I'm gonna play oldies!

[Duducrazy]

i usually go for GZDoom, but i've been also playing with PrBoom+. i find it to be a nice source port, specially when used as a replacement for Doom95.

[ReX]

And yes, I never play any mods. The only "modded" thing that I approve of in my computer is hi-res textures and something that enables higher resolutions.

Nowadays, the word "mod" loosely refers to any change to the original games. In other words, even a "simple" add-on map would be considered a mod.

For a repository of most publicly-released mods, check out Doomworld's idgames archive.

If you're looking only for levels, sort through the idgames levels. Note that there are many mods in there that are designed specifically for use with a source port (such as ZDooM). These mods are usually too detailed for regular (or "vanilla") DooM, or contain features not supported by regular DooM. If you don't want to play these, you can skip the mods here or here. Also, some mods with new levels also contain new graphics, enemies, etc. You can read through the text files that accompany the mods to determine if they contain these "extras".

For a list of mods released between 1994 and 2003 that are generally considered high quality, check out Doomworld's 10 Years of DooM. Most of the mods there, particularly from the early years, are designed for "vanilla" DooM.

For a list of mods released since 2004 that are generally considered high quality, check out Doomworld's Cacowards. Many of the mods there are designed for "vanilla" DooM.